a) Field of the Invention
The present invention relates to an analog-to-digital converter circuit and, more particularly, an analog-to-digital converter circuit adapted for use in a motor control circuit, for example.
b) Background of the Related Art
Some motor control circuits use a microprocessor and are controlled by software. FIG. 46 shows a conventional motor control circuit which is controlled by software as described above. A speed instruction voltage that is an analog signal is passed through a low-pass filter 101 to filter out noise and other higher harmonics. The analog output signal from the filter 101 is converted into digital form by an analog-to-digital converter IC 102 and supplied to a microprocessor 103 to control the output frequency of an inverter 104, thereby controlling the rotational speed of an electric motor 105. The rotational speed of the motor 105 is detected by an encoder 106. The microprocessor 103 calculates the difference between the speed instructed by the speed instruction signal and the actual speed of the motor 105, and controls the inverter 104 in such a way that the difference is reduced to zero.
The aforementioned motor control circuit is expensive because it uses an analog-to-digital converter IC. Also, it is necessary that the analog-to-digital converter IC be connected with the microprocessor by a number of lines, e.g., 10 or 12 lines, depending on the bit number. Therefore, the input/output ports of the microprocessor are occupied by the lines extending from the A/D converter IC. This makes it impossible to effectively utilize the microprocessor.